I used Logisim years ago, and what made it useful was it would allow you to program in the propagation delay in the gates to more closely simulate a real system. At the time I bought a copy with manual for $30 (now about 8 years ago). Nice to see the tool is still around ( and likely updated).

One more for your consideration: SIGROK signal analyzer. Sigrok is an analysis engine with PulseView doing the GUI. Sigrok uses over a dozen front ends, from the built-in sound cards reading analog signal at 20kHz up to 192 kHz in some cases, to dedicated hardware GPIO boards such as the cheap $8 EZ-USB boards (16 digital I/Os at 24 MHz).

Thanks a lot for bringing the collection of very nice and cool Free list of simulators many of them were not know to me, but now I will try many of them and will promote them as well.

I would like to recommend the users the site http://www.osalt.com/, at any time if you are in need of a paid software but do not want to spend you can look for the open source alternative on this site.

I was looking Calculatoredge. This is indeed a good tool for high school and engineering students. But it will be more helpful if these tools can be ported in SmartPhone (like an andriod app). Also besides calculation if it can explain the logic behind the calculation then it will be great.

I am a fun of free DIY tools and I am aware of several tools that are helpful and can be found for free. Nowadays there are many useless tools available on net and will not help you at all. They may be useless in the sense that they cannot posses the feature you are looking forward to use at that time. Some tools are worthy and are very effective. I think when designing a DIY tool, before you post it on net for free, you should make sure that it is perfect and can help in as many ways as possible.

EasyEDA is a free web-based EDA tool. It's still under heavy development and has a few rough edges but is pretty powerful stuff. It's quite unusual too because although it offers a very low cost PCB service, there's no tie-in them: users can download Gerbers and have their PCBs made by any supplier.